The present invention relates to buckets for turbines and particularly relates to a cooling system for cooling the platforms interfacing between the bucket airfoils and bucket roots.
Over the years, gas turbines have trended towards increased inlet firing temperatures to improve output and engine efficiencies. As gas path temperatures have increased, bucket platforms have increasingly exhibited distress including oxidation, creep and low cycle fatigue cracking. With the advent of closed circuit steam cooling, e.g., in the first two stages of buckets and nozzles in industrial gas turbines, inlet profiles have become such that the platforms are exposed to temperatures close to peak inlet temperatures for the blade row. This exacerbates the potential distress on bucket platforms as they run hotter.
Many older bucket designs did not require active cooling of the platforms due to lower firing temperatures. Also, film cooling carryover from upstream nozzle side walls tended to lower the temperatures near the platforms from the resulting “pitch line bias” of the inlet temperature profile. Certain designs have utilized film cooling by drilling holes through the platform and using compressor discharge air to provide a layer of cooler insulating film on the platform surface, protecting it from the high gas flow path temperatures. This is limited to areas where there is sufficient pressure to inject the film, and many current designs have insufficient pressure to film cool the entirety of the platform. Consequently, there is a need for a cooling system which will reduce the platform temperature to a level required to meet part-life or durability requirements including oxidation, creep and low cycle fatigue cracking in steam or air-cooled buckets for gas turbines.